13.24: mass spectrometry - vanderbilt...
TRANSCRIPT
1
Chapter 13: Spectroscopy Methods of structure determination
• Nuclear Magnetic Resonances (NMR) Spectroscopy (Sections 13.3-13.19) • Infrared (IR) Spectroscopy (Sections 13.20-13.22) • Ultraviolet-visible (UV-Vis) Spectroscopy (Section 13.23) • Mass (MS) spectrometry (not really spectroscopy) (Section 13.24)
Molecular Spectroscopy: the interaction of electromagnetic radiation (light) with matter (organic compounds). This interaction gives specific structural information.
2
13.24: Mass Spectrometry: molecular weight of the sample formula
The mass spectrometer gives the mass to charge ratio (m/z), therefore the sample (analyte) must be an ion.
Mass spectrometry is a gas phase technique- the sample must be “vaporized.”
Electron-impact ionization
Sample Inlet10-7 - 10-8 torr
R-H
electron beam 70 eV
(6700 KJ/mol)
e _
R-H+ mass
analyzerm/z
ionization chamber
(M+)
proton 1.00728 u neutron 1.00866 u electron 0.00055 u
1
3
mass m charge z = = B2 r2
2V
B= magnetic field strength r = radius of the analyzer tube V= voltage (accelerator plate)
The Mass Spectrometer
Ionizationchamber
Ions of selectedmass/charge ratio
are detected
Ions of non-selectedmass/charge ratioare not detected
Magnetic Field, Bo
4
Exact Masses of Common Natural Isotopes
Isotope mass natural abundance 1H 1.00782 99.985 2H 2.01410 0.015 12C 12.0000 98.892 13C 13.0033 1.108 (1.11%) 14N 14.00307 99.634 15N 15.00010 0.366 (0.38%) 16O 15.99491 99.763 17O 16.99913 0.037 (0.04%) 18O 17.99916 0.200 (0.20%)
Isotope mass natural abundance
19F 18.99840 100.00 35Cl 34.96885 75.77 37Cl 36.96590 24.23 (32.5%) 79Br 78.91839 50.69 81Br 80.91642 49.31 (98%) 127I 126.90447 100.00
Dalton (Da) or mass unit (u) = units for measuring molecular masses. One Da. = 1/12 the mass of the 12C atom
Monoisotopic (exact) mass – sum of the exact masses of the most abundant isotope of each element in a molecule
Average mass – sum of the averaged masses of each element in a molecules, weighted according to isotopic abundance.
Nominal mass – mass calculated using the integer mass of the most abundant isotope for each element (H=1, C=12, O=16, N=14, etc.)
2
5
Molecular Ion (parent ion, M) = molecular mass of the analyte; sample minus an electron. (mass of an e– is 1/1836 that of a proton) Base peak- largest (most abundant) peak in a mass spectrum; arbitrarily assigned a relative abundance of 100%.
C6H6m/z = 78.04695
m/z=78 (M+) (100%)
m/z=79 (M+1) (~ 6.6% of M+)
6
The radical cation (M+•) is unstable and will fragment into smaller ions
Rel
ativ
e ab
unda
nce
(%) m/z=16 (M+)
m/z=15
m/z=14 m/z=17 (M+1)
CH
HH + H+
m/z = 15charge neutralnot detected
CH+
H H
charge neutralnot detectedm/z = 14
CH
HH H
- e_CH
HH H
+
m/z = 16
+
Rel
ativ
e ab
unda
nce
(%)
m/z
m/z
m/z=15
m/z=29
m/z=43
m/z=45 (M+1)
m/z=44 (M)
CH
HC CH
HH
H
HH C
H
HC CH
HH
H
HH
+
CH
HC CH
HH
H
H+ H+
charge neutral notdetected
m/z = 43m/z = 44
CH
HC CH
HH
H
HH
- e_
+CH
HCH
HH + C
H
HH+
CH
HCH
HH + C
H
HH+
m/z = 29
m/z = 15charge neutralnot detected
charge neutralnot detected
- e_
3
7 m/z
m/z
Cl
Br
m/z=112 (M+)
m/z=113 (M+ +1)
m/z=114 (M+ +2)
m/z=115 (M+ +3)
m/z=77
35Cl 34.96885 75.77 37Cl 36.96590 24.23 (32.5%)
79Br 78.91839 50.69 81Br 80.91642 49.31 (98%)
m/z=77
m/z=156 (M+)
m/z=158 (M+ +2)
m/z=157 (M+ +1)
m/z=159 (M+ +3)
Mass spectrum of halogenated organic compounds
8
Mass spectra can be quite complicated and interpretation difficult. Some functional groups have characteristic fragmentations It is difficult to assign an entire structure based only on the mass spectrum. However, the mass spectrum gives the mass and formula of the sample, which is very important information. To obtain the formula, the molecular ion must be observed. (Soft ionization techniques) Methods have been developed to get large molecules such as polymers and biological macromolecules (proteins, peptides, nucleic acids) into the vapor phase
4
9
13.25: Molecular Formula as a Clue to Structure Nitrogen rule: In general, “small” organic molecules with an odd mass must have an odd number of nitrogens. Organic molecules with an even mass have zero or an even number of nitrogens. If the mass can be determined accurately enough, then the molecular formula can be determined (high-resolution mass spectrometry). Information can be obtained from the molecular formula: Degrees of unsaturation: the number of rings and/or π-bonds in a molecule (Index of Hydrogen Deficiency).
10
Degrees of unsaturation saturated hydrocarbon CnH2n+2 cycloalkane (1 ring) CnH2n alkene (1 π-bond) CnH2n alkyne (2 π-bonds) CnH2n-2
For each ring or π-bond, –2H from the formula of the saturated alkane
HH
HH
HH
H H
HH
HH
C6H14 - C6H12 H2
2 = 1 1 2
C6H14 - C6H6 H8
8 = 4 1 2
HH
HH
H
H
Hydrogen Deficiency
Degrees of Unsaturation
5
11
Correction for other elements:
For Group VII elements (halogens): subtract 1H from the H-deficiency for each halogen,
For Group VI elements (O and S): No correction is needed
For Group V elements (N and P): add 1H to the H-deficiency for each N or P
C12H4O2Cl4
C10H14N2
12
13.1: Principles of molecular spectroscopy: Electromagnetic radiation
λ = distance of one wave ν = frequency: waves per unit time (sec-1, Hz) c = speed of light (3.0 x 108 m • sec-1) h = Plank’s constant (6.63 x 10-34 J • sec)
Electromagnetic radiation has the properties of a particle (photon) and a wave.
organic molecule
(ground state)
light hν
organic molecule
(excited state)
organic molecule
(ground state)
+ hν relaxation
6
13
h•c λ
Quantum: the energy of a photon
E = hν ν =
E =
c λ
λ (cm)
short high high
Wavelength (λ) Frequency (ν)
Energy (E)
long low low
E α ν E α λ-1 ν α λ-1
1010-2
radiowavesmicrowaves
10-4
infraredVis
10310-7
UltravioletX-Raysγ Rays
10-11 10-510-9
14
13.2: Principles of Molecular Spectroscopy: Quantized Energy Levels
molecules have discrete energy levels (no continuum between levels)
A molecule absorbs electromagnetic radiation when the energy of photon corresponds to the difference in energy between two states
7
15
UV-Vis: valance electron transitions - gives information about π-bonds and conjugated systems
Infrared: molecular vibrations (stretches, bends)
- identify functional groups Radiowaves: nuclear spin in a magnetic field (NMR)
- gives a map of the H and C framework
organic molecule
(ground state)
light hν
organic molecule
(excited state)
organic molecule
(ground state)
+ hν relaxation
16
13.23 Ultraviolet-Visible (UV-Vis) Spectroscopy
λ 200
UV
400 800 nm
Vis
Recall bonding of a π-bond
8
17
π-molecular orbitals of butadiene
Ψ1: 0 Nodes 3 bonding interactions 0 antibonding interactions BONDING MO
Ψ2: 1 Nodes 2 bonding interactions 1 antibonding interactions BONDING MO
Ψ3: 2 Nodes 1 bonding interactions 2 antibonding interactions ANTIBONDING MO
Ψ4: 3 Nodes 0 bonding interactions 3 antibonding interactions ANTIBONDING MO
ψ2 is the Highest Occupied Molecular Orbital (HOMO) ψ3 is the Lowest Unoccupied Molecular Orbital (LUMO)
18
UV-Vis light causes electrons in lower energy molecular orbitals to be promoted to higher energy molecular orbitals. HOMO LUMO
Chromophore: light absorbing portion of a molecule
Butadiene
Butadiene
9
19
Molecular orbitals of conjugated polyenes
Bond
ing
Antib
ondi
ng
Energy
H2C CH2
180 nm 217 nm 258 nm 290 nm
20
violet 400 nm yellow blue 450 orange blue-green 500 red yellow-green 530 red-violet yellow 550 violet orange 600 blue-green red 700 green
Color of absorbed light λ
Color observed
380 nm 780 nm400 nm 450 nm 500 nm 550 nm 600 nm 700 nm
redorangeyellowgreenblueviolet-indigo
Molecules with extended conjugation move toward the visible region
10
21
NN
NN Mg
O
CO2CH3
O
O
Chlorophyll
β-carotene
lycopene
OHO
OH
OH
O
OH
OH
OHOOH
OHOH
+
anthocyanin
Many natural pigments have conjugated systems
22
Chromophore: light absorbing portion of a molecule Beer’s Law: There is a linear relationship between
absorbance and concentration
A = ε c l A = absorbance c = concentration (M, mol/L) l = sample path length (cm) ε = molar absorptivity (extinction coefficient) a proportionality constant for a specific absorbance of an analyte
11
23
13.20: Introduction to Infrared Spectroscopy
E α 1 λ
λ is expressed as ν (wavenumber), reciprocal cm (cm-1) _ ν = 1
λ E α ν _
therefore
_
λ (cm)
Vis NearIR
FarIRInfrared (IR) microwave
2.5 x 10-4 cm2.5 µm
1.6 x 10-3 cm16 µm
10-4 10-2
ν_
4000 ν_
600
IR radiation causes changes in a molecular vibrations
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Symmetric stretch Antisymmetric stretch
Stretch: change in bond length http://www2.chem.ucalgary.ca/Flash/photon.html
in-plane bend out-of-plane bend scissoring rocking wagging twisting
Bend: change in bond angle
>
>> >
>
> >
>
>
>
>
> >
>
>
>
Animation of bond streches and bends: http://wetche.cmbi.ru.nl//organic/vibr/methamjm.html
12
25
Bond Stretch: Hooke’s Law
ν =_
2 π c1 mx my
mx + my
f12
X Y
ν = vibrational frequencyc = speed of lightmx = mass of Xmy = mass of Y
_
mx mymx + my
= reduced mass (µ)
f = spring constant; type of bond between X and Y (single, double or triple)E α ν α f
_
Hooke’s law simulation: http://www.learnerstv.com/animation/animation.php?ani=57&cat=chemistry http://www.learnerstv.com/animation/animation.php?ani=56&cat=chemistry
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13.21 Infrared Spectra
Interpretation of an Infrared Spectra: Organic molecules contain many atoms. As a result, there are many stretching and bending modes and the IR spectrum has many absorption bands Four distinct regions of an IR spectra
4000 cm-1 600 cm-11500 cm-1
fingerprintregion
doublebondregion
2000 cm-12500 cm-1
triplebondregion
X-Hsingle bond
region
C-H O-H N-H
C≡C C≡N
C=C C=O
500 cm-1
13
27
Fingerprint region (500 - 1500 cm-1): low energy single bond stretching and bending modes. The fingerprint region is unique for any given organic compound. However, there are few diagnostic absorptions.
Double-bond regions (1600 - 1900 cm-1) C=C 1620 - 1680 cm-1 C=O 1680 - 1850 cm-1
Triple-bond region: (2100 - 2300 cm-1) C≡C 2100 - 2200 cm-1 (weak, often not observed) C≡N 2240 - 2280 cm-1
X-H Single-bond region (2800 - 3600 cm-1) O-H 3200 - 3600 cm-1 (broad) CO-OH 2500 - 3600 cm-1 (very broad) N-H 3350 - 3500 cm-1 C-H 2800 - 3300 cm-1 sp3 –C-H 2850 - 2950 cm-1 sp2 =C-H 3000 - 3100 cm-1 sp ≡C-H 3310 - 3320 cm-1
28
Alkenes =C-H 3000 - 3100 cm-1 medium - strong C=C 1620 - 1680 cm-1 medium
Aromatic =C-H 3000 - 3100 cm-1 strong C=C 1450 - 1600 cm-1 strong
Alkynes ≡C-H 3310 - 3320 cm-1 strong C≡C 2100 - 2200 cm-1 weak - medium
Alcohols C-O 1025 - 1200 cm-1 strong O-H 3200 - 3600 cm-1 strong and broad
Amines C-N 1030 - 1230 cm-1 medium N-H 3350 - 3500 cm-1 medium
Carbonyl C=O 1680 - 1850 cm-1 strong
Carboxylic acids O-H 2500 - 3500 cm-1 strong and very broad
Nitrile C≡N 2240 - 2280 cm-1 weak-medium
13.22 Characteristic Absorption Frequencies Table 13.3, p. 552
14
29
cm-1
C-H ≡C-H
C≡C
C C HH3CH2CH2CH2C
% tr
ansm
ittan
ce
% tr
ansm
ittan
ce
cm-1
C-H
C≡C
C C CH3H3CH2CH2C
C-H hexane
cm-1
% tr
ansm
ittan
ce
C-H C=C
=C-H
% tr
ansm
ittan
ce
cm-1
H3CH2CH2CH2CHC
CH
H
% tr
ansm
ittan
ce
cm-1
C=C
C-H =C-H
C CH3CH3CH2CH2C
H
H
30
O-H C-H C-O
CH3(CH2)4CH2OH
CH3(CH2)4CH2NH2
N-H
C-H
CH3(CH2)4CH2NH-CH3
N-H
C-H
C-H C≡N
H3C(H2C)4CH2C≡N % tr
ansm
ittan
ce
% tr
ansm
ittan
ce
15
31
OC OCH2CH3H3C(H2C)3H2C
OC OH(H3C)3CH2C
O-H
C-H C=O 1730 cm-1
OC HH3C(H2C)3H2C
C-H
OC CH3H3C(H2C)2H2C
C=O 1715 cm-1
C=O 1710 cm-1
C=O 1705 cm-1
OCH
% tr
ansm
ittan
ce
% tr
ansm
ittan
ce
32 http://as.vanderbilt.edu/chemistry/Rizzo/Chem220b/IR.pdf
4000 6003000 2000 1000
4000 6003000 2000 1000
X-H Regon(2500-4000 cm-1)
C-HO-HN-H
Triple BondRegion
(2000-2500)
C CC N
Double BondRegion
(1500-2000)Fingerprint Region(600-1500 cm-1)
C OC N
C C difficult to interpret
Alkanes, Alkyl groups C-H
Alkenes =C-H C=C
Alkynes ≡C-H C≡C
Alkyl Halides C-Cl C-Br C-I
Alcohols O-H C–O
Aromatics =C-H
2850-2950 (m-s)
3000-3100 (m)1620-1680 (m-w)
3310-3320 (s)2100-2260 (m-w)
600-800500-600500
3200-3600 (br, m-s)1025-1200 (s)
3000-3100 (m-w)
1450-1600 (m-w)
Amines N-H
Amide N-H C=O
Carbonyl Group C=O C–O
Carboxylic Acids O-H C=O
Nitriles C≡N
Nitro Group -NO2
3350-3500 (br, m)
3180-3350 (br, m-w)1680-1700 (s)
1650-1780 (s)~1200 (s)
2500-3100 (br, s)1700-1725 (s)
2240-2250 (w-m)
1540
cm-1 Functional Group cm-1
Typical IR Absorptions for Funtional Groups
Functional Group
16
33
13.3: Introduction to 1H NMR direct observation of the H’s of a molecules
Nuclei are positively charged and spin on an axis; they create a tiny magnetic field
++
Not all nuclei are suitable for NMR. 1H and 13C are the most important NMR active nuclei in organic chemistry Natural Abundance
1H 99.9% 13C 1.1% 12C 98.9% (not NMR active)
34
(a) Normally the nuclear magnetic fields are randomly oriented (b) When placed in an external magnetic field (Bo), the nuclear
magnetic field will either aligned with (lower energy) or oppose (higher energy) the external magnetic field
Fig 13.3, p. 513
17
35
The energy difference between aligned and opposed to the external magnetic field (Bo) is generally small and is dependent upon Bo Applied EM radiation (radio waves) causes the spin to flip and the nuclei are said to be in resonance with Bo
ΔE = h ν ΔE = γBo h 2 π
Bo = external magnetic field strength γ = gyromagnetic ratio
1H= 26.8 13C= 6.7
Note that is a constant and is sometimes denoted as h h 2π
+
+
+
+
36
NMR Active Nuclei: nuclear spin quantum number (I) atomic mass and atomic number
Number of spin states = 2I + 1 (number of possible energy levels) Even mass nuclei that have even number of neutrons have I = 0 (NMR inactive) Even mass nuclei that have an odd number of neutrons have an integer spin quantum number (I = 1, 2, 3, etc) Odd mass nuclei have half-integer spin quantum number (I = 1/2, 3/2, 5/2, etc)
I= 1/2: 1H, 13C, 19F, 31P I= 1: 2H, 14N I= 3/2: 15N I= 0: 12C, 16O
18
37
Continuous wave (CW) NMR Pulsed (FT) NMR
Fig. 13.5, p. 515
38
13.4: Nuclear Shielding and 1H Chemical Shift Different nuclei absorb EM radiation at different wavelength (energy required to bring about resonance)
Nuclei of a given type, will resonate at different energies depending on their chemical and electronic environment.
The position (chemical shift, δ) and pattern (splitting or multiplicity) of the NMR signals gives important information about the chemical environment of the nuclei. The integration of the signal is proportional to the number of nuclei giving rise to that signal
C CO
O C CH
H HH
H
H
HH
19
39
Chemical shift: the exact field strength (in ppm) that a nuclei comes into resonance relative to a reference standard (TMS)
Electron clouds “shield” nuclei from the external magnetic field causing them to resonate at slightly higher energy
Shielding: influence of neighboring functional groups on the electronic structure around a nuclei and consequently the chemical shift of their resonance.
SiCH3
CH3
CH3H3CTetramethylsilane (TMS); Reference standard δ = 0
for 1H NMR
δ = 7.28 ppm H–CCl3
Chemical shift (δ, ppm)
downfield lower magnetic field
less shielded (deshielded)
upfield higher magnetic field
more shielded
TMS
HC
Cl
ClCl
40
Vertical scale= intensity of the signal Horizontal scale= chemical shift (δ), dependent upon the field strength of the external magnetic field; for 1H, d is usually from 1-10 ppm
δ= =
14,100 gauss: 60 MHz for 1H (60 million hertz) ppm= 60 Hz 15 MHz for 13C 140,000 gauss: 600 MHz for 1H ppm = 600 Hz 150 MHz for 13C
ν – νTMS chemical shift in Hz no operating frequency in MHz
upfield higher magnetic field
more shielded
downfield lower magnetic field
less shielded (deshielded)
Chemical shift (δ, ppm)
TMS
N≡CCH2OCH3
δ = 3.50 ppm 3H δ = 4.20 ppm
2H
20
41
13.5: Effect of Molecular Structure on 1H Chemical Shift
less shielded more shielded
H3C-F H3C-O-CH3 (H3C)3-N H3C-CH3 (H3C)4-Si
δ 4.3 δ 3.2 δ 2.2 δ 0.9 δ 0.0
Electronegative substituents deshield nearby protons
H3C-H2C-H2C-H2C-O-CH2-CH2-CH2-CH3
δ= 1.37 3.40 0.92 1.55
The deshielding effect of a group drops off quickly with distance (number of bonds between the substituent and the proton)
42
The influence of neighboring groups (deshielding) on 1H chemical shifts is cumulative
HC
Cl
ClCl H
C
Cl
HCl H
C
Cl
HH
δ = 7.3 5.3 3.1 ppm
CH3CH2OH
HH
O
CH3CH2OH
HCl
O
CH3CH2OH
ClCl
O
δ = 2.1 4.06 5.96 ppm
21
43
Typical 1H NMR chemical shifts ranges – additional substitution can move the resonances out of the range (Fig. 13.8, p. 518)
0
0
123456
123456
789101112
12 1011 9 8 7
sat. alkanesR-H
aromatics
R≡C-HPhOH
CR2HOH
CR2XH
X= F, Cl, Br, I
RC CR2
HX
X=O, CR2
CR2RCOH
RO CR2
H
Ar CR2
H
O
1H NMR Shift Ranges
δ (PPM)
vinyl
R2NHR2N CR2
H
C CO
H
N
O
H
RCO2H
RCHO ROH
http://as.vanderbilt.edu/chemistry/Rizzo/Chem220b/NMR.pdf
44
Protons attached to sp2 and sp hybridize carbons are deshielded relative to protons attached to sp3 hybridized carbons
HH
HH
H
HC C C C
O
HH3C CH3
H
H H
HH H
δ = 9.7 7.3 5.3 2.1 0.9-1.5 ppm
Please read about ring current effects of π-bonds (Figs. 13.9-13.11, p. 519–522)
δ = 2.3 - 2.8 1.5 - 2.6 2.1-2.5 ppm
CH3C C
CH3 O
CH3
22
45
13.6: Interpreting 1H NMR Spectra
Equivalence (chemical-shift equivalence): chemically and magnetically equivalent nuclei resonate at the same energy and give a single signal or pattern
TMS
δ = 3.50 ppm 3H δ = 4.20 ppm
2H
C C O C HH
H
H
HN
46
C CH3C
H3C H
CH3
23
47
Test of Equivalence: 1. Do a mental substitution of the nuclei you are testing with an
arbitrary label (–X)
2. Ask what is the relationship of the compounds with the arbitrary label
3. If the labeled compounds are identical (or enantiomers), then the original nuclei are chemically equivalent and do not normally give rise to separate resonances in the NMR spectra
If the labeled compounds are not identical (and not enantiomers), then the original nuclei are not chemically equivalent and can give rise to different resonances in the NMR spectra
Identical, so the methyl groups are equivalent
Identical, so the protons are equivalent
CCH3C
CH3H3C
CH3CC
H3C
CH3X
CH3CC
H3C
XH3C
CH3CC
H3C
CH3H3C
XCC
X
CH3H3C
CH3
CCH3C
CH3C
CH3
HHH
CCH3C
CH3C
CH3
XHH
CCH3C
CH3C
CH3
HHX
CCH3C
CH3C
CH3
HXH
48
These are geometric isomers (not identical and not enantiomers). The three methyl groups are therefore not chemically equivalent and can give rise to different resonances
C CH3C
H3C H
CH3
CCH3C
HH3C
CH3
CCH3C
HX
CH3CC
X
HH3C
CH3
CCH3C
HX
CH3
CCX
HH3C
CH3
CCH3C
HH3C
X
CCH3C
HH3C
X
24
49
CH2CH3
HH
HH
H
50
Homotopic: equivalent Enantiotopic: equivalent Diastereotopic: non-equivalent
H3C C CH
H
Cl
HCH3
25
51
Integration of 1H NMR resonances: The area under an NMR resonance is proportional to the number of equivalent nuclei that give rise to that resonance.
The relative area under the resonances at δ= 4.20 and 3.50 is 2:3
TMS
δ = 3.50, δ 3H
δ = 4.20, 2H
C C O C HH
H
H
HN
52
13.7: Spin-Spin Splitting and 1H NMR protons on adjacent carbons will interact and “split” each others resonances into multiple peaks (multiplets)
n + 1 rule: equivalent protons that have n equivalent protons on the adjacent carbon will be “split” into n + 1 peaks.
δ= 4.1 2H
δ= 2.0 3H
δ= 1.2 3H
Resonances always split each other. The resonance at δ= 4.1 splits the resonance at δ = 1.2; therefore, the resonance at δ = 1.2 must split the resonance at δ = 4.2.
C CO
O C CH
H HH
H
H
HH
26
53
The multiplicity is defined by the number of peaks and the pattern (see Table 13.2 for common multiplicities patterns and relative intensities)
1 : 3 : 3 : 1
1 : 2 : 1
δ= 4.1 q, 2H
δ= 2.0 s, 3H
δ= 1.2 t, 3H
C CO
O C CH
H HH
H
H
HH
-CH2- -CH3-
54
The resonance of a proton with n equivalent protons on the adjacent carbon will be “split” into n + 1 peaks with a coupling constant J.
Coupling constant: distance between peaks of a split pattern; J is expressed in Hz. Protons coupled to each other have the same coupling constant J.
δ= 4.1 q, J=7.2 Hz, 2H
δ= 2.0 s, 3H
δ= 1.2 t, J=7.2 Hz, 3H
C CO
O C CH
H HH
H
H
HH
3Jab 3Jab 3Jab 3Jab3Jab
27
55
13.8: Splitting Patterns: The Ethyl Group
Two equivalent protons on an adjacent carbon will split a proton a triplet (t), three peaks of 1:2:1 relative intensity
Three equivalent protons on an adjacent carbon will split a proton into a quartet (q), four peaks of 1:3:3:1 relative intensity
C CH
H
H
HH
H
H
H
H
H
δ= 7.4-7.1, m, 5H δ= 2.6, q,
J= 7.0, 2H
δ= 1.2, t J= 7.0, 3H
C
H
H
H
H
H CH
H
H
HHC
O
δ= 3.0, q J= 7.2, 2H
δ= 1.2, t J= 7.2, 3H
δ= 8.0, 2H
δ= 7.6-7.3, m, 3H
56
13.9: Splitting Patterns: The Isopropyl Group
One proton on an adjacent carbon will split a proton into a doublet (d), two peaks of 1:1 relative intensity
Six equivalent protons on an adjacent carbon will split a proton into a septet (s), seven peaks of 1:6:15:20:15:6:1 relative intensity
δ= 3.0, s, J= 6.9, 1H
δ= 1.2, d J= 6.9, 6H
δ= 8.1, d, J= 6.1 Hz,
2H
δ= 7.4, d J= 6.1 Hz,
2H
δ= 2.9, s, J= 6.9, 1H
δ= 1.2, d J= 6.9, 6H
δ= 7.4-7.1, m, 5H
HH
H
H H
C CH3
H
CH3
HH
O2N
H H
C CH3
H
CH3
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57
δ= 1.2, s, 9H
δ= 3.9, s, 3H
δ= 7.4, d, J= 9.0 2H
δ= 8.0, d, J= 9.0 2H
13.10: Splitting Patterns: Pairs of Doublets
Fig. 13.20, p. 531
C OCH3
CCH3
H3CH3C
OH
H
HH
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13.11: Complex Splitting Patterns: when a protons is adjacent to more than one set of non-equivalent protons, they will split independently
J1-2 = 7.0 J2-3 = 16.0
J2-3 = 16.0
J1-2 = 7.0
H2 splits H3 into a doublet with coupling constant J2-3 = 16.0 H2 splits H1 into a doublet with coupling constant J1-2 = 7.0 H1 splits H2 into a doublet with a coupling constants J1-2 =7.0; H3 further splits H2
into a doublet (doublet of doublets) with coupling constants J2-3 = 16.0
CH
CH
CO
H
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59
δ= 0.9, t, J = 7.4, 3H
δ= 1.6, sextet J=7.4, 2H
δ= 2.6, t J = 7.4, 2H
J1,2
J2,3
J1,2 J2,3=
1 : 5 : 10:10 : 5 : 1
δ= 7.1-7.3, m, 5H
C C C HH
H
H H
H H
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Usually no spin-spin coupling between the O–H proton and neighboring protons on carbon due to exchange reaction The chemical shift of the -OH proton occurs over a large range (2.0 - 5.5 ppm). This proton usually appears as a broad singlet. It is not uncommon for this proton not to be observed.
13.12: 1H NMR Spectra of Alcohols
CH
O HH A
CH
O H H A+
13.13: NMR and Conformation (please read) NMR is like a camera with a slow shutter speed. What is observed is a weighted time average of all conformations present.
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Summary of 1H-1H Spin-Spin Coupling
• chemically equivalent protons do not exhibit spin-spin coupling to each other.
• the resonance of a proton that has n equivalent protons on the adjacent carbon is split into n+1 peaks (multiplicity) with a coupling constant J.
• protons that are coupled to each other have the same coupling constant
• non-equivalent protons will split a common proton independently: complex coupling.
Spin-spin coupling is normally observed between nuclei that are one, two and three bonds away. Four-bond coupling can be observed in certain situations (i.e., aromatic rings), but is not common.
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Summary of 1H-NMR Spectroscopy • the number of proton resonances equals the number of non-equivalent protons
• the chemical shift (δ, ppm) of a proton is diagnostic of the chemical environment (shielding and deshielding)
• Integration: number of equivalent protons giving rise to a resonance
• spin-spin coupling is dependent upon the number of equivalent protons on the adjacent carbon(s)
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13.14: 13C NMR Spectroscopy: Natural Abundance
1H 99.9% (I= 1/2) 12C 98.9% (I= 0) 13C 1.1% (I= 1/2)
ΔE= γBo h 2 π
Bo = external magnetic field strength γ = magnetogyric ratio
1H= 26.8 13C= 6.7
13C is a much less sensitive nuclei than 1H for NMR spectroscopy
New techniques (hardware and software) has made 13C NMR routine
• Pulsed NMR techniques (FT or time domain NMR) • Signal averaging (improved signal to noise)
OCH3C OCH3
1.1 % 1.1 % 1.1 %
Animation: http://www.chem.uky.edu/research/miller/NMR_Movie/puls_evol.gif
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Fourier Transform (FT) deconvolutes all of the FID’s and gives an NMR spectra.
Signal-averaging: pulsed NMR allows for many FID’s (NMR spectra) to be accumulated over time. These FID’s are added together and averaged. Signals (resonances) build up while the “noise” is random and cancels out during the averaging.
Enhanced signal to noise ratio and allows for NMR spectra to be collected on insensitive nuclei such as 13C and small samples.
13C-spectra of CH3CH2CH2CH2CH2OH
average of 200 scans after one scan
time
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65
Chemical shifts give an idea of the chemical and electronic environment of the 13C nuclei due to shielding and deshielding effects range: 0 - 220 ppm from TMS 13C NMR spectra will give a map of the carbon framework. The number of resonances equals the number of non-equivalent carbons.
200.3 137.1
132.8
128.5
128.0
17.8 40.5
13.9
TMS CDCl3
CO
CH2CH2CH3
137.1
132.8 128.5
128.0
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13.15: 13C Chemical Shifts Chemical Shift Range of 13C – see Table 13.2 (p. 538)
Note the carbonyl range
-20020406080100120140160180200220
-20020406080100120140160180200220
saturated alkanes
carbonyls vinyl
aromatics
nitriles
CR3RO
alkyne
OHR3C
R3C-F R3C-Cl R3C-I
R3C-Br
RC CR3
O
δ (PPM)
13C NMR Shift Ranges
esters, amides& acids
ketones &aldehydes
Ar-CR3
R2N-CR3
http://as.vanderbilt.edu/chemistry/Rizzo/Chem220b/NMR.pdf
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13.16: 13C NMR and Peak Intensities (please read) - 13C NMR peak integration are not generally useful
13.17: 13C-1H Coupling 1H-13C spin-spin coupling are usually “turned off” in the 13C spectra (broadband decoupled). However, spin-spin coupling tells how many protons are attached to the 13C nuclei. (i.e., primary, secondary tertiary, or quaternary carbon) 13.18: Using DEPT to Count Hydrogens Attached to 13C 13C spectra are usually collected with the 1H-13C coupling “turned off” (broad band decoupled). In this mode all 13C resonances appear as singlets. DEPT spectra (Distortionless Enhancement by Polarization Transfer) a modern 13C NMR spectra that allows you to determine the number of attached hydrogens.
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CH3 CH3
CH3
CH2 CH2
Broad-band decoupled
DEPT
OH
12345
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8
6 5
2 4 1
7 8
3
CH CH
CH2’s (methylenes) give negative resonances CH’s (methines) and CH3’s (methyls) give positive resonances Quaternary carbon (no attached H’s) are not observed 13.19: 2D NMR: COSY and HETCOR (please read)
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Solving Combined Spectra Problems: Mass Spectra:
Molecular Formula Nitrogen Rule → # of nitrogen atoms in the molecule M+1 peak → # of carbons Degrees of Unsaturation: # of rings and/or π-bonds
Infrared Spectra: Functional Groups C=O O-H C=C N-H C≡C CO-OH C≡N
1H NMR: Chemical Shift (δ) → chemical environment of the H's Integration → # of H's giving rise to the resonance Spin-Spin Coupling (multiplicity) → # of non-equivalent H's on the adjacent carbons (vicinal coupling).
13C NMR: # of resonances → symmetry of carbon framework Type of Carbonyl
Each piece of evidence gives a fragment (puzzle piece) of the structure. Piece the puzzle together to give a proposed structure. The proposed structure should be consistent with all the evidence.
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Problem 13.43
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71
Problem 13.44
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Problem 13.46: C5H10O 13C NMR:
212.1
35.5
7.9
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δ= 7.4-7.1 (m, 5H)
δ= 2.61 (sextet,
J=7.0, 1H)
δ= 2.61 (pentet,
J=7.0, 2H)
δ= 2.61 (d, J=7.0, 3H)
δ= 2.61 (t, J=7.0, 3H)
C10H14
147.6
128.2
127.0
125.7 41.7 31.2
21.8 12.3
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Chapter 15: Alcohols, Diols, and Thiols 15.1: Sources of Alcohols (Table 15.1, p. 616)
Hydration of alkenes (Chapter 6) 1. Acid-catalyzed hydration (Chapter 6.6) 2. Oxymercuration (p. 258-9) 3. Hydroboration (Chapter 6.8)
Hydrolysis of alkyl halides (Chapter 8.1) nucleophilic substitution
Reaction of Grignard or organolithium reagents with ketones, aldehydes, and esters. (Chapter 14.5)
Reduction of aldehydes, ketones, esters, and carboxylic acids (Chapters 15.2 - 15.3)
Reaction of epoxides with Grignard Reagents (Chapter 15.4)
Diols from the dihydroxylation of alkenes (Chapter 15.5)
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